The invention relates to an x-ray diagnostic installation comprising means for the formation of an electric signal which corresponds to the radiation transparency of the radiography subject.
In x-ray diagnostics it is important to know the radiation transparency of the patient in order to be able to prepare optimally blackened (or dense) radiographs. In the case of utilization of an x-ray automatic exposure control which automatically terminates a radiograph when a predetermined radiation dose has acted upon the x-ray film, the disconnection (or switch-off) dose, as determined by means of the switching stage of the automatic exposure timer, is dependent, due to the dependency of the sensitivity of the detector upon the radiation quality, not only upon the selected x-ray tube voltage, but, in the case of a given subject thickness, is also dependent upon the radiation transparency of the radiography subject. This dependency is accounted for in that intensifier foils are employed in conjunction with the x-ray film, which foils absorb a large portion of the x-radiation and in that the radiation measuring chamber of the automatic exposure timer is arranged behind the cassette with the x-ray film and the intensifier foils. Due to the hardening through the radiography subject, the absorption of the x-radiation by the intensifier foils, in addition to being dependent upon the x-ray tube voltage, in the case of a given subject thickness, is also dependent upon the radiation transparency of the radiography subject. Accordingly, it would be necessary to supply to the switching stage of the automatic exposure timer, for a precise exposure, information regarding the subject thickness as well as information regarding the radiation transparency of the radiography subject, which codetermine the optimum disconnection (or switch-off) dose value. The described difficulties can, indeed, be avoided if the radiation detector of the automatic exposure control is arranged in front of the photographic exposure system; in this case, however, a shadow-free radiation detector must be employed. In addition, as compared with the arrangement of the radiation detector behind the photographic exposure system, a somewhat enlarged subject-to-film distance and an increased prefiltering of the image-producing radiation results.